System, method and apparatus for target material debris cleaning of EUV vessel and EUV collector

ABSTRACT

A system and method of removing target material debris deposits simultaneously with generating EUV light includes generating hydrogen radicals in situ in the EUV vessel, proximate to the target material debris deposits and volatilizing the target material debris deposits and purging the volatilized target material debris deposits from the EUV vessel without the need of an oxygen containing species in the EUV vessel.

CLAIM FOR PRIORITY

This application is a continuation of U.S. application Ser. No.15/003,385, filed Jan. 21, 2016. The disclosure of this application fromwhich priority is claimed is incorporated by reference herein in itsentirety for all purposes.

BACKGROUND

Extreme ultraviolet (EUV) light is used in applications such as extremeultraviolet lithography (EUVL).

The extreme ultraviolet (EUV) light may be generated using an EUV sourcein which a target material is irradiated by a high power laser source.The irradiation of the target material by the laser source leads to thegeneration of plasma which emits EUV light.

An EUV collector situated in an EUV vessel collects and focuses the EUVlight emitted by the plasma. The collected EUV light is directed out ofthe EUV vessel and into an EUV consuming system such as an extremeultraviolet lithography system (EUVL).

Significant portions of the target material are distributed around theEUV vessel as target material debris when the target material isirradiated by the high power laser source. The target material debrisdeposits on the EUV collector and the various internal surfaces withinthe EUV vessel. The target material debris deposits on the EUV collectorreduce the collection performance of the EUV collector. The targetmaterial debris deposits on the internal surfaces of the EUV vessel caneventually flake off the internal surfaces and come to rest on the EUVcollector, further diminishing the collection performance of the EUVcollector.

It is within this context that embodiments arise.

SUMMARY

Broadly speaking, the present invention fills these needs by providing asystem and method for generating hydrogen radicals H* in situ in the EUVvessel, proximate to the target material debris deposits. It should beappreciated that the present invention can be implemented in numerousways, including as a process, an apparatus, a system, computer readablemedia, or a device. In situ cleaning provides the capability of cleaningof EUV collector and EUV vessel simultaneously with generating EUVlight. In situ cleaning allows the EUV generation operation is notrequired to be interrupted for cleaning the EUV collector and the EUVvessel. Several inventive embodiments of the present invention aredescribed below.

One embodiment provides a system for removing target material debrisdeposits simultaneously with generating EUV light includes generatinghydrogen radicals in situ in the EUV vessel, proximate to the targetmaterial debris deposits and volatilizing the target material debrisdeposits and purging the volatilized target material debris depositsfrom the EUV vessel without the need of an oxygen containing species inthe EUV vessel.

Another embodiment provides an EUV light source including an EUV vesselincluding an EUV vessel purge gas inlet coupled to a purge gas sourcecapable of dispensing a quantity of purge gas into the EUV vessel. AnEUV collector is disposed in the EUV vessel. The EUV collector includinga reflective surface. A target material source is capable of dispensinga quantity of target material into the EUV vessel. A first portion ofthe quantity of target material is disposed on at least a portion of thereflective surface of the EUV collector as a first target materialdebris deposit. A first hydrogen radical source is disposed within theEUV vessel. The first hydrogen radical source including a first hydrogenradical source outlet disposed proximate to the reflective surface ofthe EUV collector. The first hydrogen radical source also includes afirst hydrogen source inlet coupled to a hydrogen source, a firsthydrogen source electrode coupled to a first signal source and a secondhydrogen source electrode coupled to a second signal source. The firsthydrogen radical source being capable of producing a first quantity ofhydrogen radicals and dispensing the first quantity of hydrogen radicalsfrom the first hydrogen radical source outlet. The first quantity ofhydrogen radicals capable of combining with the first target materialdebris deposit to form a first quantity of a volatile compoundcontaining at least a portion of the first target material debrisdeposit. An EUV vessel purge outlet is included in the EUV vessel and iscapable of passing the first quantity of the volatile compound out ofthe EUV vessel.

The hydrogen radical source can include a hydrogen plasma chamber suchas a capacitively or inductively coupled hydrogen plasma chamber. One ofthe electrodes used for generating the hydrogen radicals can be aportion of a conductive layer of the EUV collector. The hydrogen radicalsource outlet can be disposed around a perimeter of the EUV collector ordisposed near a central aperture of the EUV collector. The hydrogenradical source can be disposed near one or more baffles in the EUVvessel.

The hydrogen radicals are produced proximate to the target materialdebris deposits and thus do not require an oxygen containing specie toextend the time before the hydrogen radicals recombine to form hydrogengas and thus oxygen containing specie can be prevented from entering theEUV vessel.

Another embodiment provides a method of cleaning a target materialdebris deposits in an EUV light source while simultaneously generatingEUV light in the EUV light source. The method includes generating aquantity of hydrogen radicals within an EUV vessel of the EUV lightsource and outputting the generated quantity of hydrogen radicalsproximate to the target material deposit on a surface inside the EUVvessel. A first quantity of a volatile compound containing at least aportion of the first portion the target material deposit is formed. Asufficient quantity of purge gas is dispensed into the EUV vessel andthe first quantity of the volatile compound is purged out of the EUVvessel though an EUV vessel purge outlet.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings.

FIG. 1A is a simplified schematic view of a laser-produced-plasma EUVlight source, in accordance with embodiments of the disclosed subjectmatter.

FIG. 1B illustrates a mid-vessel baffle assembly in an EUV vessel, inaccordance with embodiments of the disclosed subject matter.

FIG. 1C is a schematic diagram of the EUV collector with target materialdebris deposits, in accordance with embodiments of the disclosed subjectmatter.

FIG. 1D is a schematic diagram of a portion of the baffle assembly, inaccordance with embodiments of the disclosed subject matter.

FIG. 2 is a simplified schematic view of an EUV light source includingone or more in situ hydrogen radical sources, in accordance withembodiments of the disclosed subject matter.

FIG. 3A is a detailed cross-sectional view of a portion of the collectorand an in situ hydrogen radical source, in accordance with embodimentsof the disclosed subject matter.

FIG. 3B is a simplified schematic of a cross-sectional view of thecollector and the in situ hydrogen radical source, in accordance withembodiments of the disclosed subject matter.

FIG. 4 is a simplified schematic of a cross-sectional view of thecollector and an alternative, in situ hydrogen radical source, inaccordance with embodiments of the disclosed subject matter.

FIG. 5 is a simplified view of the collector having an in situ hydrogenradical source substantially surrounding the perimeter of the collector,in accordance with embodiments of the disclosed subject matter.

FIG. 6 is a simplified view of the collector having multiple in situhydrogen radical sources substantially evenly distributed and disposedaround the perimeter of the collector, in accordance with embodiments ofthe disclosed subject matter.

FIG. 7 is a simplified schematic of a cross-sectional view of thecollector and multiple in situ hydrogen radical sources, in accordancewith embodiments of the disclosed subject matter.

FIG. 8 is a simplified schematic side view of an inductive hydrogenradical generator, in accordance with embodiments of the disclosedsubject matter.

FIG. 9 is a simplified schematic top view of the inductive hydrogenradical generator, in accordance with embodiments of the disclosedsubject matter.

FIG. 10 is a simplified schematic side view of an inductive hydrogenradical generator, in accordance with embodiments of the disclosedsubject matter.

FIG. 11 is a simplified schematic side view of a capacitive hydrogenradical generator, in accordance with embodiments of the disclosedsubject matter.

FIG. 12 is a simplified schematic side view of a capacitive hydrogenradical generator, in accordance with embodiments of the disclosedsubject matter.

FIG. 13 is a simplified schematic side view of the EUV vessel includingcapacitive hydrogen radical sources disposed in or near the baffles, inaccordance with embodiments of the disclosed subject matter.

FIG. 14 is a flowchart diagram that illustrates the method operationsperformed in generating hydrogen radicals in situ in the EUV vessel, inaccordance with embodiments of the disclosed subject matter.

FIG. 15 is a flowchart diagram that illustrates the method operationsperformed in producing EUV light while simultaneously removing targetmaterial debris deposits in the EUV vessel, in accordance withembodiments of the disclosed subject matter.

DETAILED DESCRIPTION

Several exemplary embodiments for removing target material debris fromthe EUV vessel using in-situ hydrogen radical generators will now bedescribed. It will be apparent to those skilled in the art that thepresent invention may be practiced without some or all of the specificdetails set forth herein.

Several different types of target materials can be used for generatingEUV emitting plasma. One implementation utilizes tin and/or tincompounds. Some examples of tin containing target materials include puretin, tin compounds including one or more of SnBr4, SnBr2, SnH4, and tinalloys including one or more of tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, and combinations thereof.

Unfortunately the tin containing debris can contaminate many of theinside surfaces of the EUV vessel and most specifically the EUVcollector. Several approaches have been tried in the past to remove thetin containing debris. One approach includes injecting hydrogen into theEUV vessel to convert the deposited tin to a volatile tin compound suchas SnH4 that can then be purged from the EUV vessel. Unfortunately,merely injecting hydrogen results in a relatively slow conversion of thedeposited tin to the volatile tin compounds.

In one example implementation, hydrogen radicals, including H+ and/or H−ions, can be generated using a microwave hydrogen radical generatorlocated externally from the EUV vessel. Unfortunately, by the time thehydrogen radicals arrived inside the EUV vessel, most of the hydrogenradicals had combined to form H2 hydrogen gas.

In one approach to ensure delivery of a greater number of hydrogenradicals, oxygen, and more specifically water vapor, can be injectedwith the hydrogen radicals. As a result, the numbers of hydrogenradicals that arrive within the EUV vessel is sufficient to perform thedesired combination with the deposited tin to form volatile tincompounds. However, injecting oxygen or water vapor into the EUV vesselrequires that the EUV generation process carried out within the EUVvessel must be terminated during the presence of oxygen or water vaporas the oxygen and/or water vapor causes problems with the EUV generationprocess.

One implementation disclosed herein is to increase the conversion of thedeposited tin to volatile tin compounds by generating hydrogen radicals,including H+ and/or H− ions, in a hydrogen radical source disposedin-situ in the EUV vessel. The hydrogen radical source will producehydrogen radicals in high concentration. The hydrogen radical source canbe disposed in very close proximity to the EUV collector such that theoutlet of the hydrogen radical source delivers the hydrogen radicalsdirectly to the EUV collector. The outlet of the hydrogen radical sourcecan be disposed in one or more locations around the perimeter of the EUVcollector and/or near the center of the EUV collector.

Hydrogen radical source can be a single hydrogen radical source ormultiple hydrogen radical sources. In one implementation, the hydrogenradical source is in the form of an annular shaped plasma vesseldisposed near the perimeter of the EUV collector. In anotherimplementation, the hydrogen radical source is in the form of multiplehydrogen radical sources disposed near the perimeter of the EUVcollector and/or the center of the EUV collector.

One or more additional hydrogen radical sources can also be includedwithin the EUV vessel. By way of example, additional hydrogen radicalsources can be included near the vanes in the outlet portion of the EUVvessel so as to provide hydrogen radicals proximate to the vanes so asto convert tin deposited on the vanes to the volatile tin compounds.

Generating the hydrogen radicals within the EUV vessel ensures thatsufficient numbers of hydrogen radicals are available to convert thedeposited tin to the volatile tin compounds at the same time that theEUV vessel is being used to generate EUV. As a result, the EUV lightsource can be operated for much longer periods of time before thedeposited tin interferes with the generation of the EUV. As long assufficient numbers of hydrogen radicals can be generated, the depositedtin can be substantially eliminated and maintained at a substantiallyzero level so as to alleviate the need for disassembly and cleaning ofthe EUV vessel as a result of excess tin deposits.

FIG. 1A is a simplified schematic view of a laser-produced-plasma EUVlight source 20, in accordance with embodiments of the disclosed subjectmatter. The LPP light source 20 includes a light pulse generation system22 for generating a train of light pulses and delivering the lightpulses into a EUV vessel 26. Each light pulse 23 travels along a beampath 21 inside a beam transport system 25 from the light pulsegeneration system 22. The light pulse 23 is focused into the EUV vessel26 to illuminate and/or irradiate a selected target droplet at anirradiation region 28.

Suitable lasers for use in the light pulse generation system 22 shown inFIG. 1, may include a pulsed laser device, e.g., a pulsed gas dischargeCO2 laser device producing radiation at about 9.3 μm or about 10.6 μm,e.g., with DC or RF excitation, operating at relatively high power,e.g., about 10 kW or higher and high pulse repetition rate, e.g., about10 kHz or more. In one particular implementation, the laser in the lightpulse generation system 22 may be an axial-flow RF-pumped CO2 laserhaving a MOPA configuration with multiple stages of amplification andhaving a seed pulse that is initiated by a Q-switched master oscillator(MO) with low energy and high repetition rate, e.g., capable of 100 kHzoperation. From the MO, the laser pulse may then be amplified, shaped,and focused before reaching the irradiation region 28.

Continuously pumped CO2 amplifiers may be used for the light pulsegeneration system 22. For example, a suitable CO2 laser device having anoscillator and multiple amplifiers (e.g., O-PA1-PA2 . . . configuration)is disclosed in commonly owned U.S. Pat. No. 7,439,530, filed on Jun.29, 2005 and issued on Oct. 21, 2008, entitled, LPP EUV LIGHT SOURCEDRIVE LASER SYSTEM, the entire contents of which are hereby incorporatedby reference herein.

Alternatively, the laser in the light pulse generation system 22 may beconfigured as a so-called “self-targeting” laser system in which thesurface of the target material in the laser waist serves as one mirrorof the optical cavity. In some “self-targeting” arrangements, a masteroscillator may not be required. Self-targeting laser systems aredisclosed and claimed in commonly owned U.S. Pat. No. 7,491,954, filedOct. 26, 2005 and issued Feb. 17, 2009, entitled, DRIVE LASER DELIVERYSYSTEMS FOR EUV LIGHT SOURCE, the entire contents of which are herebyincorporated by reference herein.

Depending on the application, other types of lasers may also be suitablefor use in the light pulse generation system 22, e.g., an excimer ormolecular fluorine laser operating at high power and high pulserepetition rate. Other examples include, a solid state laser, e.g.,having a fiber, rod or disk shaped active media, a MOPA configuredexcimer laser system, e.g., as shown in commonly owned U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450, the entire contents of which arehereby incorporated by reference herein, an excimer laser having one ormore chambers, e.g., an oscillator chamber and one or more amplifyingchambers (with the amplifying chambers in parallel or in series), amaster oscillator/power oscillator (MOPO) arrangement, a masteroscillator/power ring amplifier (MOPRA) arrangement, a poweroscillator/power amplifier (POPA) arrangement, or a solid state laserthat seeds one or more excimer or molecular fluorine amplifier oroscillator chambers, may be suitable. Other light source designs arepossible.

Referring again to FIG. 1A, the EUV light source 20 may also include atarget material delivery system 24, for delivering portions (e.g.,droplets) of a target material into the interior of a EUV vessel 26 tothe irradiation region 28, where the droplets 102A, 102B will interactwith one or more light pulses 23, e.g., one or more pre-pulses andthereafter one or more irradiating pulses, to ultimately produce aplasma and a corresponding emission of EUV light 34. The unused orunirradiated droplets 102C are collected in a target material catch 200.The target material may include, but is not necessarily limited to, amaterial that includes tin, lithium, xenon, etc., or combinationsthereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., maybe in the form of liquid droplets and/or solid particles containedwithin liquid droplets 102A, 102B or other forms as described elsewhereherein.

By way of example, the element tin may be used as pure tin, as a tincompound, e.g., SnBr4, SnBr2, SnH4, as a tin alloy, e.g., tin-galliumalloys, tin-indium alloys, tin-indium-gallium alloys, or a combinationthereof. Depending on the material used, the target material may bepresented to the irradiation region 28 at various temperatures includingroom temperature or near room temperature (e.g., tin alloys, SnBr4), atan elevated temperature, (e.g., pure tin) or at temperatures below roomtemperature, (e.g., SnH4), and in some cases, can be relativelyvolatile, e.g., SnBr4. More details concerning the use of thesematerials in an LPP EUV light source is provided in commonly owned U.S.Pat. No. 7,465,946, filed on Apr. 17, 2006 and issued Dec. 16, 2008,entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, the contents of whichare hereby incorporated by reference herein.

Referring further to FIG. 1A, the EUV light source 20 includes a EUVcollector 30. The EUV collector 30 is a near-normal incidence EUVcollector having a reflective surface in the form of a prolate spheroid(i.e., an ellipse rotated about its major axis). The actual shape andgeometry can of course change depending on the size of the chamber andthe location of focus. The EUV collector 30 can include a gradedmulti-layer coating in one or more embodiments. The graded multi-layercoating can include alternating layers of molybdenum and silicon, and insome cases one or more high temperature diffusion barrier layers,smoothing layers, capping layers and/or etch stop layers.

The EUV collector 30 also includes an aperture 32. The aperture 32allows the light pulses 23 generated by the light pulse generationsystem 22 to pass through to the irradiation region 28. The EUVcollector 30 can be a prolate spheroid mirror that has a primary focus31 within or near the irradiation region 28 and an intermediate focus40. The EUV light 34 is output at or near the intermediate focus 40 fromthe EUV light source 20 and input to a downstream device 42 utilizingEUV light 34. By way of example, the downstream device 42 that receivesthe EUV light 34 can be an integrated circuit lithography tool (e.g., ascanner).

It is to be appreciated that other optics may be used in place of theprolate spheroid mirror, e.g., EUV collector 30, for collecting anddirecting EUV light 34 to the intermediate focus 40 for subsequentdelivery to a device utilizing the EUV light. By way of example the EUVcollector 30 can be a parabola rotated about its major axis.Alternatively, the EUV collector 30 can be configured to deliver a beamhaving a ring-shaped cross-section to the location of the intermediatefocus 40 (e.g., commonly owned U.S. Pat. No. 7,843,632, filed on Aug.16, 2006 and issued Nov. 30, 2010, entitled EUV OPTICS, the contents ofwhich are hereby incorporated by reference).

The EUV light source 20 may also include a EUV controller 60. The EUVcontroller 60 can include a firing control system 65 for triggering oneor more lamps and/or laser devices in the light pulse generation system22 to thereby generate light pulses 23 for delivery into the chamber 26.

The EUV light source 20 may also include a target material positiondetection system including one or more target material imagers 70. Thetarget material imagers 70 can capture images using CCD's or otherimaging technologies and/or backlight stroboscopic illumination and/orlight curtains that provide an output indicative of the position and/ortiming of one or more target material droplets 102A, 102B relative tothe irradiation region 28. The imagers 70 are coupled to and output thetarget material location and timing data to a target material positiondetection feedback system 62. The target material position detectionfeedback system 62 can compute a target material position andtrajectory, from which a target material position error can be computed.The target material position error can be calculated on each portion oftarget material or an average basis (e.g., on a droplet by droplet basisor on average droplet data). The target material position error may thenbe provided as an input to the EUV controller 60. The EUV controller 60can provide a position, direction and/or timing correction signal to thelight pulse generation system 22 to control a source timing circuitand/or to control a beam position and shaping system to change thetrajectory and/or focal power or focal point of the light pulses 23being delivered to the irradiation region 28 in the chamber 26.

The EUV light source 20 can also include one or more EUV metrologyinstruments for measuring various properties of the EUV light generatedby the source 20. These properties may include, for example, intensity(e.g., total intensity or intensity within a particular spectral band),spectral bandwidth, polarization, beam position, pointing, etc. For theEUV light source 20, the instrument(s) may be configured to operatewhile the downstream tool, e.g., photolithography scanner, is on-line,e.g., by sampling a portion of the EUV output, e.g., using a pickoffmirror or sampling “uncollected” EUV light, and/or may operate while thedownstream tool, e.g., photolithography scanner, is off-line, forexample, by measuring the entire EUV output of the EUV light source 20.

The EUV light source 20 can also include a target material controlsystem 90, operable in response to a signal (which in someimplementations may include the target material position error describedabove, or some quantity derived there from) from the EUV controller 60,to e.g., modify the release point of the target material from a targetmaterial dispenser 92 and/or modify target material formation timing, tocorrect for position errors in the target material droplets 102A, 102Barriving at the desired irradiation region 28 and/or synchronize thegeneration of target material droplets 102A, 102B with the light pulsegeneration system 22.

Additional details and alternatives of the EUV light source 20 are alsodescribed in commonly owned U.S. Pat. No. 8,575,575, filed on Mar. 16,2010 and issued on Nov. 5, 2013 and entitled “System, Method andApparatus for Laser Produced Plasma Extreme Ultraviolet Chamber with HotWalls and Cold Collector Mirror”, which is incorporated by reference inits entirety. U.S. Pat. No. 8,575,575 provides implementations where theEUV collector 30 and other internal surfaces such as the vanes arecooled to a temperature less than the melting point of the tin depositsformed on the surfaces of the EUV collector as in some implementationsthe solid form tin deposits are more easily converted to the volatiletin compounds.

Additional details and alternatives of the EUV light source 20 are alsodescribed in commonly owned U.S. Pat. No. 8,653,491, filed on Mar. 16,2010 and issued on Feb. 18, 2014 and entitled “System, Method andApparatus for Aligning and Synchronizing Target Material for OptimumExtreme Ultraviolet Light Output”, which is incorporated by reference inits entirety. U.S. Pat. No. 8,653,491 provides implementations for moreaccurately targeting the target material to improve the amount of EUVemitting plasma.

FIG. 1B illustrates a mid-vessel baffle assembly 150 in an EUV vessel26, in accordance with embodiments of the disclosed subject matter. Thebaffle assembly 150 is located in a mid-vessel region 155′ of the EUVvessel 26. The secondary region 155 of the EUV vessel 26 is divided intotwo portions: the mid vessel region 155′ and an aft vessel region 155″.The mid vessel region 155′ begins at the irradiation region 28 andextends toward the outlet 40A of the EUV vessel 26. The aft vesselregion 155″ extends between the mid vessel region 155′ and the outlet40A of the EUV vessel 26. The mid vessel region 155′ and the aft vesselregion 155″ have no specific length and therefore the mid vessel region155′ can include substantially all of the secondary region 155 of theEUV vessel 26, in some implementations.

The baffle assembly 150 includes a series of passages and structuresthat receive, slow and capture a portion of the microparticles 153created when target material is irradiated in the irradiation region 28.The baffle assembly 150 can be formed from a series of vanes or otherstructures and porous materials that extend from the irradiation region28 and the EUV collector 30 to the location of the intermediate focus 40or any portion of the secondary region 155 of the EUV vessel 26. Whilethe baffle assembly 150 can extend from the irradiation region 28 andthe EUV collector 30 to the intermediate focus 40, the baffle assemblydoes not prevent or otherwise occlude the EUV light 34 from passing fromthe EUV collector 30 through a three dimensional, cone-shapedtransmissive region 152 to the intermediate focus 40.

The passages in the baffle assembly 150 begin at the edges 154A, 154B ofthe transmissive region 152 and the passages in the baffle assembly 150extend to the inner surfaces 156 of the EUV vessel 26. The baffleassembly 150 can include a series of concentric baffles surrounding butnot protruding into the transmissive region 152. The baffle assembly 150extends substantially from the edges 154A, 154B of the transmissiveregion 152 to the inner surfaces 156 of the EUV vessel 26.

It should be noted that while shown in a horizontal configuration, insome implementations, the EUV vessel 26 is configured and a nearvertical orientation such that baffle assembly 150 and the outlet 40A ofthe EUV vessel are oriented substantially directly above the collector30. As a result target material deposits forming on the baffle assembly150 can become dislodged and be inadvertently collected on the collector30.

FIG. 1C is a schematic diagram of the EUV collector 30 with targetmaterial debris deposits 161A-D and 162, in accordance with embodimentsof the disclosed subject matter. The target material debris deposits161A-D and 162 can be present in many forms. By way of example,relatively small particulates 161A, can deposit on the surface of thecollector 30 that are discernible by the naked eye. Larger deposits161B, 161D can include multiple small particulates that coagulate orotherwise accumulate and/or aggregate into larger deposits and/or targetmaterial debris deposits that initially formed on the baffle assembly150 and were released from the baffle assembly and subsequentlydeposited on the collector 30. Even larger target material debrisdeposits 161C can also form for various reasons on the collector 30. Inaddition to the called out target material debris deposits 161A-D, avery fine layer of target material debris deposits 162 can coatsubstantially the entire surface of the collector 30 and other surfacesinternal to the EUV vessel 26. The very fine layer of target materialdebris deposits 162 can be made up of substantial microscopic targetmaterial debris such as might resemble a substantially uniform coatingof dust on the surface of the collector 30.

FIG. 1D is a schematic diagram of a portion of the baffle assembly 150,in accordance with embodiments of the disclosed subject matter. Thebaffle assembly 150 includes many separate vanes 151A, 151B. The vanes151A, 151B are separated from one another by varying distances asillustrated. Further the vanes are formed at varying angles asillustrated by the differences between vanes 151A and vanes 151B. Targetmaterial debris deposits 171A-171C can form at various locations on themany separate vanes 151A, 151B. By way of example, target materialdeposits 171A can form near the region of the intermediate focus 40 ofthe EUV vessel 26. Similarly, target material debris deposits 171B and171C can form along the outer edges of the vanes 151A, 151B, somewhatcloser to the inner surfaces 156 of the EUV vessel 26. As noted abovethe target material debris deposits 171A-171C can form initially on thebaffle assembly 150 and then for various reasons become dislodged fromthe baffle assembly and be collected on the surface of the collector 30.

The various target material debris deposits 161A-161D, 171A-171Cultimately interfere in various ways with the performance of the EUVvessel 26 and must be removed at some point. One approach to removingthe target material debris deposits is to interrupt the generation ofEUV and disassemble the EUV vessel 26 to clean each of the individualparts of the EUV vessel such as the collector 30, the baffle assembly150 and other inner surfaces 156. However, interrupting the generationof the EUV in the EUV vessel 26 also results in interruptions to the EUVlithography process that is consuming the EUV generated in the EUVvessel, effectively ceasing production. A more effective EUV vesselcleaning process is needed. Various forms of in situ hydrogen radicalgeneration provides a much more effective and timely cleaning processfor removing the various target material to pre-deposits 161A-161D,171A-171C from the EUV vessel 26, without interrupting generation of theEUV light in the EUV vessel.

FIG. 2 is a simplified schematic view of an EUV light source 200including one or more in situ hydrogen radical sources 201, 201′, 201″,in accordance with embodiments of the disclosed subject matter. The EUVlight source 200 includes one or more in situ hydrogen radical sources201, 201′, 201″ that can be placed in one or more locations within theEUV vessel 26. In situ hydrogen radical sources 201 are placed near theperimeter of the collector 30. Central in situ hydrogen radical sources201′ are placed near the center aperture 32 of the collector 30. Bafflein situ hydrogen radical sources 201″ are placed near the baffleassemblies of the EUV vessel 26. It should be noted that the EUV vessel26 can include as few as one of the in situ hydrogen radical sources201, 201′, 201″ and as many as can be physically placed within the EUVvessel. The in situ hydrogen radical sources 201, 201′, 201″ can be oneor more of the different types and configurations of in situ hydrogenradical sources as will be described in more detail below.

A hydrogen gas source 290 is coupled to each of the in situ hydrogenradical sources 201, 201′, 201″. A carrier gas source 291 can optionallybe coupled to each of the in situ hydrogen radical sources 201, 201′,201″. As will be described in more detail below a signal source 212 orsources can be coupled to each of the in situ hydrogen radical sources201, 201′, 201″. It should be noted that the in situ hydrogen radicalsources 201, 201′, 201″ can be placed in substantially symmetricalpositions or asymmetrical positions within the EUV vessel 26.

FIG. 3A is a detailed cross-sectional view of a portion of the collector30 and an in situ hydrogen radical source 201, in accordance withembodiments of the disclosed subject matter. FIG. 3B is a simplifiedschematic of a cross-sectional view of the collector 30 and the in situhydrogen radical source 201, in accordance with embodiments of thedisclosed subject matter. The collector 30 has a collector surface 30Aand a collector rim 30B. The collector 30 also includes a centralaperture 32. The hydrogen radical source 201 is disposed adjacent to thecollector 30. In the shown embodiment, the hydrogen radical source 201is disposed immediately adjacent to the rim 30B of the collector 30.

The hydrogen radical source 201 includes a radical generator 202, anoutlet channel 203 that leads to an over the rim channel 204 and anoutlet 205. The radical generator 202 is coupled to a signal source 212.The hydrogen radical source 201 is coupled to a hydrogen source that isnot shown. The hydrogen source can be a source of hydrogen gas or otherhydrogen containing source material such as a hydrogen containing gas ora mixture of hydrogen containing gases. The hydrogen source can alsoinclude a mixing apparatus for mixing the hydrogen containing gases withan inert carrier gas 208 such as argon, helium, nitrogen, and othersubstantially inert get carrier gases. In another implementation, theinert carrier gas 208 can be injected directly into the hydrogen radicalsource 201.

Hydrogen radicals H*, including H+ and/or H− ions are created byinjecting hydrogen into the hydrogen radical source 201 and energizingthe radical generator 202. The hydrogen radicals then flow through theoutlet channel 203 through the over the rim channel 204 and out theoutlet 205 which is proximate to the surface 30A of the collector 30.The inert carrier gas can be used to transport the hydrogen radicalsfrom the hydrogen radical source 201 to the surface 30A of the collector30. The hydrogen radicals can then react with the target material debrisdeposits 171A, 162 disposed on the surface 30A of the collector 30 tocreate the volatile tin compounds. The volatile tin compounds can thenbe purged from the EUV vessel 26 through the purge outlet 296 (shown inFIG. 2) using a purge gas source 295 (shown in FIG. 2).

Referring to FIG. 3B, the hydrogen radical source 201 can be acapacitively coupled hydrogen plasma chamber having walls 201A coupledto a first potential and the radical generator 202 coupled to a secondpotential. As shown, the walls 201A are grounded and the radicalgenerator 202 is coupled to a signal source 210. However it should beunderstood that the walls 201A could be coupled to the signal source 212and the radical generator 202 could be grounded or coupled to a secondsignal source. In one implementation the signal source 212 can be an RFsignal source having a frequency in the range of 10s of kHz to about 10GHz. The signal source 212 can excite the hydrogen present in thehydrogen radical source 201 to generate a hydrogen plasma 202A thatdissociates the hydrogen into the hydrogen radicals H*.

FIG. 4 is a simplified schematic of a cross-sectional view of thecollector 30 and an alternative, in situ hydrogen radical source 221, inaccordance with embodiments of the disclosed subject matter. Thealternative, in situ hydrogen radical source 221 is somewhat similar toin situ hydrogen radical source 201, however includes a ceramicinsulator 220 to isolate the walls 201A and the radical generator 211 ofthe alternative, in situ hydrogen radical source 221 from the plasma202A generated therein.

FIG. 5 is a simplified view of the collector 30 having an in situhydrogen radical source 231 substantially surrounding the perimeter ofthe collector, in accordance with embodiments of the disclosed subjectmatter. The in situ hydrogen radical source 231 is an annular chamberdisposed around the perimeter of the collector 30. The in situ hydrogenradical source 231 has a cross-section substantially similar to in situhydrogen radical sources 201 or 221, as described above. The in situhydrogen radical source 231 has an outlet 205A around the perimeter ofthe collector 30 such that hydrogen radicals H* are generated and outputsubstantially evenly around the perimeter of the collector.

FIG. 6 is a simplified view of the collector 30 having multiple in situhydrogen radical sources 201 substantially evenly distributed anddisposed around the perimeter of the collector, in accordance withembodiments of the disclosed subject matter. Each of the in situhydrogen radical sources 201 include an outlet 205 such that thehydrogen radicals H* generated by each of the hydrogen radical sourcesare output around the perimeter of the collector 30. Each of the in situhydrogen radical sources 201 can be any one or more of the in situhydrogen radical sources described herein. In one implementation each ofthe hydrogen radical sources 201 that are distributed around theperimeter of the collector 30 are the same type of hydrogen radicalsource. In another implementation the hydrogen radical sourcesdistributed around the perimeter of the collector 30 include one or moretypes of hydrogen radical sources.

FIG. 7 is a simplified schematic of a cross-sectional view of thecollector 30 and multiple in situ hydrogen radical sources 201, inaccordance with embodiments of the disclosed subject matter. One or moreof the multiple in situ hydrogen radical sources 201 are disposed aroundthe perimeter and along the edge 30B of the collector 30. In addition tothe in situ hydrogen radical sources 201 disposed around the perimeterof the collector 30, one or more central in situ hydrogen radicalsources 271 are disposed proximate to the center aperture 32 such thathydrogen radicals H* generated therein are output through the centeraperture of the collector. The hydrogen radicals H* generated in centralin situ hydrogen radical sources 271 provide hydrogen radicals proximateto the target material debris deposited on the collector 30 and near thecenter aperture 32. It should be noted that the central in situ hydrogenradical sources 271 are shown to resemble hydrogen radical source 201described above, however each of the central in situ hydrogen radicalsources 271 can be any sort of hydrogen radical source described herein.

Each of the central in situ hydrogen radical sources 271 has arespective signal source 212A and 212B and the in situ hydrogen radicalsource 201 has a respective signal source 212. The respective signalsources 212, 212A, 212B can be the same or different signal sourceshaving the same or different frequencies, powers, duty cycles, oramplitudes so that each of the in situ hydrogen radical sources 201, 271can be individually controlled to produce the quantity of hydrogenradicals as may be needed for each local surface area having targetmaterial debris deposited thereon. By way of example, if the area of thesurface of the collector 30 proximate to the aperture 32 has a largequantity of target material debris deposited thereon, and the area nearthe perimeter of the collector has relatively small quantity of targetmaterial to debris deposited thereon, then the quantity of hydrogenradicals H* needed near the aperture would be greater than the quantityof hydrogen radicals H* needed near the perimeter to remove therespective amounts of target material debris local to each of theoutlets of the in situ hydrogen radical sources 201, 271.

FIG. 8 is a simplified schematic side view of an inductive hydrogenradical generator 800, in accordance with embodiments of the disclosedsubject matter. FIG. 9 is a simplified schematic top view of theinductive hydrogen radical generator 800, in accordance with embodimentsof the disclosed subject matter. The inductive hydrogen radicalgenerator 800 includes multiple hydrogen nozzles 802 disposed around theperimeter of the collector 30. An inductive coil 804 is disposed betweenthe hydrogen nozzles and the rim 30B of the collector 30.

The inductive coil 804 has a first end coupled to a signal source 212C.The inductive coil 804 has a second end coupled to ground. It should benoted that the inductive coil 804 is shown making only one substantialloop around the perimeter of the collector 30, however this is shown tosimplify the description of the inductive coil and that the inductivecoil can include one or more loops around the perimeter of thecollector.

The signal source 212C outputs and induction signal of suitablefrequency amplitude and duty cycle. As the induction signal passesthrough the induction coil 804, a magnetic field is induced into thecenter of the coil as shown in FIG. 9. The hydrogen nozzles 802 toinject hydrogen into the center of the coil and a hydrogen plasma 810can be created. The hydrogen plasma generates the hydrogen radicalsH*proximate to the surface 30A of the collector 30 where the hydrogenradicals H* can interact with any target material deposits that may bepresent.

FIG. 10 is a simplified schematic side view of an inductive hydrogenradical generator 1000, in accordance with embodiments of the disclosedsubject matter. The inductive hydrogen radical generator 1000 differsfrom the inductive hydrogen radical generator 800 described above inFIGS. 8 and 9 in that the induction coil 1004 is disposed external to asidewall 1008 of the EUV vessel 26. The sidewall 1008 includes a ceramicwindow 1007 through which the induction coil can induce the magneticfield over the surface 30A of the collector 30. As described above thehydrogen nozzles 802 inject hydrogen into the magnetic field generatedby the induction coil 1004 to create a hydrogen plasma 810 that producesthe needed hydrogen radicals H*.

FIG. 11 is a simplified schematic side view of a capacitive hydrogenradical generator 1100, in accordance with embodiments of the disclosedsubject matter. The capacitive hydrogen radical generator 1100 utilizesthe conductive layer 1120 of the collector 30 as a first electrode andthe walls 1008 of the EUV vessel 26 as the second electrode. Thecollector 30 includes multiple layers 1120, 1122, 1124, and 1126. One ofthe layers is a conductive layer 1120. The conductive layer 1120 can beformed from any suitable conductive material including copper, aluminum,steel, stainless steel and combinations and alloys containing copper,aluminum, steel, stainless steel. The conductive layer 1120 can alsoinclude heating and cooling devices and subsystems for managing thetemperature of the collector 30. In one implementation the conductivelayer 1120 includes resistive heaters and/or cooling channels forpassing a cooling fluid such as a gas or liquid coolant. The conductivelayer 1120 can also provide physical structural support and mountingpoints for the collector 30.

A silicon, glass or quartz layer 1122 is supported by the conductivelayer 1120. A reflective layer 1124 is supported on the silicon, glassor quartz layer 1122. The reflective layer 1124 performs the reflectivefunction of the collector 30. An optional protective layer 1126 can beformed on top of the reflective layer 1124. Should be understood thatthe thicknesses of the different layers 1120, 1122, 1124 and 1126 arenot accurately shown in the detailed view of FIG. 11.

In operation, applying a signal from signal source 212C to theconductive layer 1120 capacitively couples the signal into the hydrogengas injected from the hydrogen nozzle 1106 to create a hydrogen plasma1110 that generates the hydrogen radicals H* near the surface 30A of thecollector 30.

FIG. 12 is a simplified schematic side view of a capacitive hydrogenradical generator 1200, in accordance with embodiments of the disclosedsubject matter. The hydrogen radical source 221 can be a capacitivelycoupled hydrogen plasma chamber having walls 201A coupled to a firstpotential and the radical generator 211 coupled to a second potential.As shown, the walls 201A are grounded and the radical generator 202 iscoupled to a signal source 212. However it should be understood that thewalls 201A could be coupled to the signal source 212 and the radicalgenerator 211 could be grounded or coupled to a second signal source(not shown). In one implementation the signal source 212 can be an RFsignal source having a frequency in the range of 10s of kHz to about 10GHz. The signal source 212 can excite the hydrogen present in thehydrogen radical source 221 to generate a hydrogen plasma 202A thatdissociates the hydrogen into the hydrogen radicals H*.

The capacitive hydrogen radical generator 1200 also utilizes theconductive layer 1120 of the collector 30 as a third electrode that iscoupled to second signal source 1212. The second signal source 1212 canproduce a signal ranging from DC (0 Hz) to as much as 10s of MHz. In oneimplementation the second signal source 1212 outputs a signal frombetween 0 Hz to about 13 MHz. The second signal source 1212 can be usedto pull the hydrogen radicals H* toward the surface 30A of the collector30 and thereby increase the concentration of the hydrogen radicals H*near the surface 30A and the target material debris deposits thereon.Increasing the concentration of the hydrogen radicals H* near thesurface 30A increases the conversion of the tin in the target materialdebris deposits to volatile tin compounds such as SnH4.

FIG. 13 is a simplified schematic side view of the EUV vessel 26including capacitive hydrogen radical sources 201″ disposed in or nearthe baffles 150, in accordance with embodiments of the disclosed subjectmatter. The hydrogen radical sources 201″ include a first electrode 1302and a second electrode 1304. The first electrode 1302 is grounded, insome implementations. The second electrode 1304 is coupled to a signalsource 1312, in some implementations. Alternatively, the first electrode1302 can be coupled to the signal source 1312 and the second electrode1304 coupled to ground. Alternatively, in lieu of coupling one of theelectrodes 1302, 1304 to ground, that electrode could be coupled to asecond signal source, not shown. The second electrode 1304 can beinsulated from the side wall 1008 of the EUV vessel 26 by an optionalinsulating layer 1306.

Injecting hydrogen into the hydrogen radical sources 201″ and applyingappropriate signals to the electrodes 1302, 1304, generates the hydrogenradicals H* proximate to the baffles 150. The hydrogen radicals H* canthen react with the target material debris deposits on the baffles 150to produce a volatile byproduct that can be evacuated or purged from theEUV vessel 26.

FIG. 14 is a flowchart diagram that illustrates the method operations1400 performed in generating hydrogen radicals in situ in the EUV vessel26, in accordance with embodiments of the disclosed subject matter. Theoperations illustrated herein are by way of example, as it should beunderstood that some operations may have sub-operations and in otherinstances, certain operations described herein may not be included inthe illustrated operations. With this in mind, the method and operations1400 will now be described.

In an operation 1405, hydrogen radicals are generated in situ in the EUVvessel proximate to target material debris deposits. Hydrogen radicalsH*are generated by the various hydrogen radical sources described above.Several different hydrogen radical sources have been described above andmore than one hydrogen radical sources can be included within the EUVvessel 26, however the hydrogen radical sources can be selectivelyactivated to remove certain target material debris deposits that areproximate to the activated hydrogen radical source or sources. It shouldbe noted that each of the hydrogen radical sources can be operatedindependently and at different operating parameters to produce varyingquantities of hydrogen radicals H*.

In operation 1410, the hydrogen radicals H*combine with the targetmaterial debris deposits to convert the target material debris depositsto a volatile compound. In an operation 1415, the volatile compound isremoved from the EUV vessel 26. The volatile compound can be removed theEUV vessel 26 by purging or by evacuating the volatile compound. Itshould be noted that operations 1405 through 1415 can be conducted whilethe EUV vessel 26 is also producing EUV.

In one implementation, the in situ cleaning process includes generatingand injecting hydrogen radicals near the aperture 32 at a flow rate ofabout 90 slm and generating and injecting hydrogen radicals near theperimeter of the EUV collector 30 at a flow rate of about 90 slm whilemaintaining the pressure at about 1.3 Torr and the signal sources 212,212A, 212B, 212C, 1212 providing a signal of between about several kHzto 100s of MHz RF at a power of between about 1 kW to several kWdepending on the desired cleaning rate. It should be understood thatthese are merely example flow rates, RF frequency and RF power and thatlesser or greater flow rates, RF frequencies and RF powers andcombinations thereof can be utilized. It should also be understood thatthe hydrogen radical generation and injection can be a different flowrate, RF frequency, and RF power in one or more of the local portions ofthe perimeter of the EUV collector 30 and the aperture 32 as may beneeded to address additional target material debris proximate to therespective outlets of each of the hydrogen radical sources.

FIG. 15 is a flowchart diagram that illustrates the method operations1500 performed in producing EUV light while simultaneously removingtarget material debris deposits in the EUV vessel 26, in accordance withembodiments of the disclosed subject matter. The operations illustratedherein are by way of example, as it should be understood that someoperations may have sub-operations and in other instances, certainoperations described herein may not be included in the illustratedoperations. With this in mind, the method and operations 1500 will nowbe described.

In an operation 1505, the source laser is directed to a portion oftarget material within an EUV vessel 26.

In operation 1510, a plasma is generated when the source laser interactswith the target material in the EUV vessel 26. When the source laserinteracts with the target material in the EUV vessel, a first portion ofthe target material is converted to the plasma and a second portion ofthe target material is cast off as target material debris.

In an operation 1515, EUV light emitted from the plasma is collected inthe collector 30. The second portion of the target material settles onvarious inner surfaces of the EUV vessel 26, in an operation 1520.

In operation 1525, if additional EUV light needs to be generated, thenthe method operations continue in operation 1505 as described above. Ifno additional EUV light is needed to be generated than the methodoperations can end.

Simultaneously, with operation 1505, hydrogen radicals H* are generatedin situ in the EUV vessel 26, in an operation 1507. Hydrogen radicalsare generated in one or more hydrogen radical sources disposed withinthe EUV vessel 26.

In operation 1512, the hydrogen radicals H* interact with the targetmaterial debris deposits to form a volatile compound. As described abovetypically, the target material contains tin and the volatile compoundcreated with the interaction of hydrogen radicals H* is SnH4. It shouldbe understood that any suitable volatile compound containing at least aportion of the target material debris deposits can be created inoperation 1512.

In operation 1517, the volatile compound created in operation 1512 isremoved from the EUV vessel 26. The volatile compound can be removed bypurging or by evacuating the EUV vessel 26 and combinations thereof.

In operation 1522 if additional target material debris deposits need tobe removed then the method operations continue in operation 1507 above.If no additional target material debris deposits need to be removed fromthe EUV vessel 26 and the method operations can end. It should be notedthat the target material debris deposits removal in operations 1507 to1522 can be conducted simultaneously with generation of the EUV inoperations 1505 to 1525 in the EUV vessel 26. In other implementations,the target material debris deposits removal in operations 1507 to 1522can be conducted in an overlapping or an alternating fashion withgeneration of the EUV in operations 1505 to 1525.

In one implementation, the in situ cleaning process includes generatingand injecting hydrogen radicals in various locations around the EUVvessel at a flow rate of about 90 slm while maintaining the pressure atabout 1.3 Torr and the signal source 1312 providing a signal of betweenabout several kHz to 100s of MHz RF at a power of between about 1 kW toseveral kW depending on the desired cleaning rate. It should beunderstood that these are merely example flow rates, RF frequency and RFpower and that lesser or greater flow rates, RF frequencies and RFpowers and combinations thereof can be utilized. It should also beunderstood that the hydrogen radical generation and injection can be adifferent flow rate, RF frequency, and RF power in one or more of thelocal portions of the EUV vessel, as may be needed to address additionaltarget material debris proximate to the respective outlets of each ofthe hydrogen radical sources disposed around the interior of the EUVvessel.

With the above embodiments in mind, it should be understood that theinvention may employ various computer-implemented operations involvingdata stored in computer systems. These operations are those requiringphysical manipulation of physical quantities. Usually, though notnecessarily, these quantities take the form of electrical or magneticsignals capable of being stored, transferred, combined, compared, andotherwise manipulated. Further, the manipulations performed are oftenreferred to in terms, such as producing, identifying, determining, orcomparing.

Any of the operations described herein that form part of the inventionare useful machine operations. The invention also relates to a device oran apparatus for performing these operations. The apparatus may bespecially constructed for the required purpose, such as a specialpurpose computer. When defined as a special purpose computer, thecomputer can also perform other processing, program execution orroutines that are not part of the special purpose, while still beingcapable of operating for the special purpose. Alternatively, theoperations may be processed by a general purpose computer selectivelyactivated or configured by one or more computer programs stored in thecomputer memory, cache, or obtained over a network. When data isobtained over a network the data maybe processed by other computers onthe network, e.g., a cloud of computing resources.

The embodiments of the present invention can also be defined as amachine that transforms data from one state to another state. Thetransformed data can be saved to storage and then manipulated by aprocessor. The processor thus transforms the data from one thing toanother. Still further, the methods can be processed by one or moremachines or processors that can be connected over a network. Eachmachine can transform data from one state or thing to another, and canalso process data, save data to storage, transmit data over a network,display the result, or communicate the result to another machine.

The invention can also be embodied as computer readable code on acomputer readable medium. The computer readable medium is any datastorage device that can store data, which can thereafter be read by acomputer system. Examples of the computer readable medium include harddrives, network attached storage (NAS), read-only memory, random-accessmemory, CD-ROMs, CD-Rs, CD-RWs, DVDs, Flash, magnetic tapes, and otheroptical and non-optical data storage devices. The computer readablemedium can also be distributed over a network coupled computer systemsso that the computer readable code is stored and executed in adistributed fashion.

It will be further appreciated that the instructions represented by theoperations in the above figures are not required to be performed in theorder illustrated, and that all the processing represented by theoperations may not be necessary to practice the invention. Further, theprocesses described in any of the above figures can also be implementedin software stored in any one of or combinations of the RAM, the ROM, orthe hard disk drive.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. An extreme ultraviolet (EUV) light sourcecomprising: an EUV vessel having an EUV vessel purge gas inlet, the EUVpurge gas inlet being configured to be coupled to a purge gas source fordispensing a quantity of purge gas into the EUV vessel; an EUV collectordisposed in the EUV vessel, the EUV collector having a reflectivesurface; a target material source for dispensing a quantity of targetmaterial into the EUV vessel; a hydrogen radical source disposed withinthe EUV vessel, the hydrogen radical source comprising a capacitivelycoupled hydrogen plasma chamber, the capacitively coupled hydrogenplasma chamber comprising: a hydrogen radical source outlet disposedproximate to the reflective surface of the EUV collector; a hydrogensource inlet configured to be coupled to a hydrogen source; a hydrogensource electrode configured to be coupled to a signal source anddisposed within the EUV vessel for generating a capacitively coupledplasma during operation of the EUV light source to produce hydrogenradicals that combine with at least a portion of a target materialdebris deposit disposed on the reflective surface of the EUV collectorto generate a volatile compound; and an EUV vessel purge outlet forpassing the volatile compound out of the EUV vessel.
 2. The EUV lightsource of claim 1, wherein the EUV collector has a central aperture andthe hydrogen radical source outlet is disposed proximate to the centralaperture so that hydrogen radicals generated during operation of the EUVlight source pass through the central aperture.
 3. The EUV light sourceof claim 1, wherein the EUV collector has a central aperture and thehydrogen radical source is a first hydrogen radical source disposedproximate to the central aperture, and the EUV light source furthercomprises: a second hydrogen radical source disposed proximate to thecentral aperture, each of the first and second hydrogen radical sourcesdisposed proximate to the central aperture so that hydrogen radicalsgenerated during operation of the EUV light source pass through thecentral aperture.
 4. An extreme ultraviolet (EUV) light source,comprising: an EUV vessel having an EUV vessel purge gas inlet and anEUV vessel purge gas outlet, the EUV vessel purge gas inlet beingconfigured to be coupled to a purge gas source for supplying a quantityof purge gas into the EUV vessel; an EUV collector disposed in the EUVvessel, the EUV collector having a reflective surface; a target materialdispenser for supplying a quantity of target material into the EUVvessel; and a hydrogen radical source disposed within the EUV vessel,the hydrogen radical source being defined by a capacitively coupledhydrogen plasma chamber having at least one grounded wall and anelectrode coupled to a signal source, the hydrogen radical source isconfigured to receive a flow of hydrogen and produce hydrogen radicalsproximate to the reflective surface of the EUV collector when the signalsource is active during operation, wherein during operation the hydrogenradicals combine with a portion of target material disposed on thereflective surface to remove the portion of target material as avolatile compound, the EUV vessel purge gas outlet used to purge thevolatile compound from the EUV vessel.
 5. The EUV light source of claim4, wherein the hydrogen radical source is disposed within the EUV vesselso that hydrogen radicals are provided at a perimeter of the EUVcollector during operation.
 6. The EUV light source of claim 5, whereinthe hydrogen radical source is disposed within the EUV vessel so thathydrogen radicals are provided at a perimeter of the EUV collectorduring operation, wherein the EUV collector has a central aperture, andwherein the EUV light source comprises an additional hydrogen radicalsource disposed behind the EUV collector so that hydrogen radicals areprovided near the central aperture of the EUV collector duringoperation.
 7. The EUV light source of claim 4, wherein the EUV collectorhas a central aperture, and the hydrogen radical source is disposedbehind the EUV collector so that hydrogen radicals are provided near thecentral aperture of the EUV collector during operation.
 8. The EUV lightsource of claim 4, wherein additional hydrogen radical sources areprovided to define a plurality of hydrogen radical sources disposedwithin the EUV vessel, the plurality of hydrogen radical sources beingdisposed within the EUV vessel so that hydrogen radicals are providedaround a perimeter of the EUV collector during operation.
 9. The EUVlight source of claim 8, wherein the plurality of hydrogen radicalsources are symmetrically disposed around the perimeter of the EUVcollector.
 10. The EUV light source of claim 8, wherein the EUVcollector has a central aperture, and wherein the EUV light sourcecomprises an additional hydrogen radical source disposed behind the EUVcollector so that hydrogen radicals are provided near the centralaperture of the EUV collector during operation.